1. Field of the Invention
The present invention generally relates to data storage and data transfer techniques, and particularly relates to a storage apparatus and a data transfer method capable of using storages at different transfer rates in a network.
2. Description of the Related Art
Recently, there has been a remarkable development in information technologies relating to computers and communications, and vast amounts of data are being handled. To store vast amounts of data, such storages as a plurality of hard disk devices or magnetic tape devices are used while being connected to one another. A system called storage area network (SAN) is constituted by connecting these storages to a server or the like so as to be able to efficiently store and manage vast amounts of data.
In the SAN, data is transferred at transfer rate in gigabits according to a protocol such as Fiber Channel Protocol (FCP). Examples of the topology of the SAN include a fiber channel-arbitrated loop (FC-AL) with which the storages are connected into a loop. All the storages in the loop need to have the same transfer rate. It is, therefore, impossible to add a hard disk device having a transfer rate of, for example, 2 gigabits per second (Gbps) into a loop of hard disk devices each having a transfer rate of 4 Gbps.
One possible approach to solve the disadvantage is to add only a hard disk device having a transfer rate of 4 Gbps to the loop of hard disk devices having the transfer rate of 4 Gbps. Another possible approach is to reduce the transfer rate of the hard disk devices in the loop from 4 Gbps to 2 Gbps before adding the hard disk device having a transfer rate of 2 Gbps. However, with the former approach, the degree of freedom of selection is small because of the restriction on the transfer rate of the hard disk device that can be added. With the latter approach, throughput is reduced due to the reduction in transfer rates of all the hard disk devices.
The following technique is described in, for example, Japanese Patent Application Laid-Open No. 2004-348464. A plurality of storages at different transfer rates is mixed in one loop by using a predetermined circuit that shapes signals passing in the loop to make frequencies of the signals on a communication path coincident with one another.
However, various processes such as a process for comparing phases of the signals are required to shape the signals in the loop, which disadvantageously increases the size of the circuit. That is, the storages having different transfer rates transmit or receive signals at different frequencies because of disparity in transfer rate. To make the different frequencies coincident, it is necessary to calculate a phase difference among the signals transmitted and received by the respective storages, and to reduce this difference to be close to zero. As a result, the signal shaping requires many circuits such as a phase comparator and an oscillator, and increases the size of the entire circuit.
Moreover, the signal shaping requires some processing time. Due to this, if a storage having a different transfer rate is added into the loop, a time lag is generated before the entire loop is actually operable.